Recently, there has been an increasing interest in energy storage technology. As the application fields of energy storage technologies have been extended to mobile devices such as cellular phones, camcorders and notebook computers, the demand for electrochemical devices as a power source has been increasing. Among electrochemical devices, lithium secondary batteries have been commercially available and well used by virtue of its high energy density and voltage, long cycle life, and low self-discharge rate.
Also, recently, with the growing interest in environmental issues, many studies are conducted on electric vehicles (EVs) and hybrid electric vehicles (HEVs) other than vehicles running on fossil fuels, such as gasoline vehicles and diesel vehicles being attributable to air pollution. These electric vehicles and hybrid electric vehicles largely use Ni-metal hydride secondary batteries, and also lithium secondary batteries having high energy density and voltage, long cycle life and low self-discharge rate are actively researched for their use and have reached commercialization.
In such a lithium secondary battery, carbon materials have been largely used as a cathode active material, and also the use of metallic lithium or sulfur-containing compounds has been considered. In addition, lithium-containing cobalt oxides (LiCoO2), as well as LiMnO2 of a layered crystal structure, lithium-containing manganese oxides such as LiMn2O4 of a spinel crystal structure, and lithium-containing nickel oxides (LiNiO2) have been used as a cathode active material.
Among these, LiCoO2 having excellent life characteristics and good charging/discharging efficiency has been used the most, but has disadvantages in terms of low structural safety, and high price due to resource limits of cobalt as a raw material which results in low price competitiveness. Accordingly, LiCoO2 is insufficient to use in large amounts as a power source in the field of industries such as electric vehicles.
Meanwhile, LiNiO2 as a cathode active material is relatively inexpensive and has high discharge capacity, but undergoes rapid phase transition in its crystal structure due to volume change generated during charge/discharge cycles. Also, when LiNiO2 is exposed to air or moisture, its safety is rapidly lowered.
Lithium manganese oxides such as LiMnO2 and LiMn2O4 have advantages of good thermal stability and inexpensiveness, but are unfavorable in terms of low capacity, and poor cycle and high-temperature characteristics.
Among the lithium manganese oxides, spinel LiMn2O4 exhibits relatively even potential around 4V (between 3.7 and 4.3 V) and around 3V (between 2.5 and 3.5 V) and has a theoretical capacity of about 120 mAh/g at such areas. However, spinel LiMn2O4 exhibits poor cycle and storage characteristics around 3V, making it difficult to be applied. This is because of phase transition induced by Jahn-Teller distortion, that is, LiMn2O4 exists in a single phase of a cubic phase around 4V and it exists in two phases of the cubic phase and a tetragonal phase around 3V. Another reason is the release of manganese into an electrolytic solution. Above all, the contraction and expansion of volume may cause a short circuit between a conductive material or a polymer binder and a current collector.
For these reasons, the lithium manganese oxides having a spinel structure exhibit an actual capacity lower than the theoretical capacity and low C-rate characteristics around 3V.
Accordingly, it is known that the lithium manganese oxides having a spinel structure are difficult to be used around 3V, and in order to solve this problem, there are some attempts of forming a tetragonal phase and S-doping to improve cycle characteristics around 3V. However, such an attempt has provided insignificant effect or has not surely proven the reason of improvement.
Therefore, there is still a need for developing a spinel-structured lithium manganese oxide having good capacity and superior life characteristics around 3V.